Accompanying the recent trend to higher density of optical disks, there have come to be used the objective lenses which have higher numerical apertures (NA) and are incorporated in optical pickup apparatuses used in recording/reproducing for optical disks, for the demand for smaller convergent spots.
For example, in the optical pickup apparatus for high density optical disks using a violet semiconductor laser light source having wavelength λ of 405 nm, it has been suggested to use an objective lens having numerical aperture NA of 0.85, for attaining higher densities.
As an objective lens having a numerical aperture of 0.85, the Unexamined Patent Application Publication (hereinafter, referred to as JP-A) No. H10-123410 discloses a two-group lens made of glass wherein NA of 0.85 has been attained with relaxed manufacture tolerance for each lens, by making two lenses to share refracting power for an incident light flux.
However, in the case of injection molding for an aspherical lens made of glass, it is necessary to raise a temperature of a die to about 550° C., which causes a problem that a die for an aspherical lens made of glass is generally short in terms of its life, and it is not suitable for mass production.
Further, since a protective layer having a thickness of 0.1 mm is used in the high density optical disk using a laser light flux with wavelength 405 nm and an objective lens with NA 0.85, a scratch on the surface of the protective layer has a great influence on characteristics of recording/reproducing of information. Therefore, securing a working distance in design of the objective lens is extremely important for preventing that the objective lens interferes with the optical disk to scratch the surface of the protective layer.
However, in the case of an objective lens of a two-group construction, a height of a marginal beam from an optical axis is small in the case of passing through an optical surface facing the optical disk, and a distance (working distance) from the objective lens to the optical disk is short, because an incident light flux is converged on an information recording surface of an optical disk after being refracted by two lenses respectively, which makes the objective lens to interfere easily with the optical disk. In particular, on the objective lens of a two-group type made of glass which is described in the above patent application, a possibility for the objective lens to interfere with an optical disk and thereby to scratch the surface of the protective layer is extremely high, because a weight of the objective lens is great.
To deal with the aforesaid problem, therefore, one of the inventors of the invention proposed a plastic single lens having NA 0.85 that is like one described in JP-A No. 2001-324673.
Compared with a glass lens, a lens made of plastic can be subjected to injection molding at lower temperature (approximately 120° C.), and therefore, the expected life span of a die for the plastic lens is longer, and expenses of materials for the plastic lens are less expensive. Therefore, the plastic lenses can be manufactured on a mass production basis at low cost, while keeping stable quality. Further, owing to the structure of a single lens, a long working distance can be secured, and a problem of damage on the surface of a protective layer caused by an interference between an objective lens and an optical disk can be solved because of light weight.
Though the plastic single lens with NA 0.85 has the aforementioned advantageous point, in comparison with a glass lens of a two-group type, it has a problem that spherical aberration caused by changes in refractive index resulting from temperature changes grows greater. A basis for the foregoing is that changes in spherical aberration caused on a plastic single lens are increased in proportion to NA4.
Incidentally, in the following explanation, characteristics of the optical element in the case of temperature changes are called “temperature characteristics”, and correcting wavefront aberration changes of an optical element accompanying prescribed temperature changes so that the wavefront aberration changes may be not more than the diffraction limit is called “correction of temperature characteristics”.
In the optical pickup apparatus, laser power for recording of information is generally greater than that for reproducing of information, which sometimes causes mode hopping which is a phenomenon that a central wavelength of a laser light source is made by changes in output to jump several microns instantaneously when reproducing is switched to recording. Though a deviation of a focus position caused by the mode hopping can be removed by focusing of an objective lens, troubles such as poor recording caused by the deviation of a focus position are raised for a period of several nanoseconds until focusing of the objective lens. Since this deviation of a focus position grows greater when a wavelength of a light source is shorter, deterioration of wavefront aberration caused by mode hopping grows greater when a wavelength of a light source is shorter. From the reasons stated above, correction of the deviation of a focus position of a convergent spot due to wavelength changes has been demanded, especially in the optical pickup apparatus for high-density optical disks employing a violet semiconductor laser as a light source.
Incidentally, in the following explanation, “chromatic aberration” means a longitudinal chromatic aberration and/or a variation in spherical aberration, which are generated on the optical element due to a fluctuation in wavelength of an incident light flux. Wavefront aberration is deteriorated when a focus position of a convergent spot formed by an optical element is deviated by the chromatic aberration of this kind. “Correction of chromatic aberration” means that “chromatic aberration” of the optical element caused by prescribed fluctuation in wavelength of the incident light flux is corrected to be a diffraction limit or less.
Since plastic materials capable of being used in a short wavelength area are limited, it is impossible to control this chromatic aberration to be small by selecting materials having large Abbe's number. Further, compared with a two-group type lens, in a single lens, an amount of occurrence of the chromatic aberration for the same wavelength change of the incident light flux tend to grow greater.
Namely, for using a plastic single lens with NA 0.85 as an objective lens of an optical pickup apparatus that employs a violet semiconductor laser as a light source, it is desirable to conduct “correction of chromatic aberration” as well as “correction of temperature characteristics”.
As a technology to correct temperature characteristics of a plastic single lens, JP-A H11-337818 discloses a technology wherein there is formed, on an optical surface of the plastic single lens, a diffractive structure having wavelength-dependency of spherical aberration in which the spherical aberration changes in the direction toward insufficient correction when an incident light flux changes to be longer.
Further, International Publication Pamphlet No. 02/41307 discloses a technology wherein chromatic aberration and temperature characteristics are corrected by providing a diffractive structure and a structure of plural steps extending in the optical axis direction (NPS: non-periodic phase structure) on optical surfaces of the plastic single lens.
However, in the plastic single lens on which the diffractive structure described in JP-A No. H11-337818 is formed, spherical aberration changes toward excessive correction due to refractive index decline of the objective lens by temperature rise is cancelled and corrected by spherical aberration changes toward insufficient correction by a diffractive structure due to the change of wavelength of the incident light by the temperature rise.
Namely, since the aforementioned technology is one to utilize characteristics that oscillation wavelength of a semiconductor laser is shifted to the longer wavelength side when temperature rises, an effect of correction of temperature characteristics cannot be obtained in the situation where only temperature of an objective optical system arranged in the vicinity of an actuator that becomes a heat source during operations of the optical pickup apparatus rises, and temperature of a semiconductor laser arranged to be away from the actuator does not change substantially, which is a problem.
In contrast to the foregoing, the technology disclosed in the International Publication Pamphlet No. 02/41307 is one to use a diffractive structure for correcting chromatic aberration and to use NPS for correcting temperature characteristics. Therefore, even in the situation where only temperature of the objective optical system rises and the temperature of the semiconductor laser does not change substantially, an effect of correction of temperature characteristics can be obtained, which is an advantageous point. However, when a structure having microscopic steps in the optical axis direction such as a diffractive structure or NPS is formed on an optical surface of an objective optical system that requires large refracting power, a course of a light flux entering a side of the step is blocked (a light flux is eclipsed). Thus, the light flux does not contribute to forming of a convergent spot and causes a decline of an amount of light. In particular, in the plastic single lens with NA 0.85, the radius of curvature of the optical surface thereof is extremely small, and therefore, an improvement of a decline of an amount of light caused by eclipse of the light flux at the step in the optical axis direction in the diffractive structure or in NPS is demanded.
Further, since the change in the spherical aberration due to the temperature characteristics caused on a plastic single lens is increased in proportion to NA4, it may be difficult to sufficiently conduct the correction of the spherical aberration of the temperature characteristics by only NPS when a lens having a very high NA, such as a single plastic lens having NA of 0.85, is used. Thus, further improvement in the efficiency of correction of the temperature characteristics has been still demanded.